Years ago Theme Magazine did an article on Namaiki, a Tokyo-based design duo advocating guerilla gardening. They'd basically drop "seed bombs" in empty urban lots, roadside tree plots, anywhere there's dirt, and let nature do the rest. Thus you'd have broccoli, radishes and even pumpkins growing in the unlikeliest of places.
Suck UK's contribution to the guerilla gardening movement is the Flower Grenade, loaded up with Buttercup, Poppy and Ryegrass seeds. Toss these over the fence into an abandoned lot and the clay shells will dissolve next time it rains. And hopefully, a week later you'll peek through the fence and see the beginnings of new life.